The present invention relates to self-powered radiation detectors which are typically utilized for in-core nuclear reactor radiation monitoring. The conventional self-powered radiation detector utilizes a central emitter wire, insulating means about the emitter wire, and a coaxially disposed collector sheath about the insulating means. The term self-powered relates to the fact that no electrical potential is required to be applied across the detector electrodes. A signal current is generated as a function of the differing radiation response characteristics of the emitter and collector electrode materials. The emitter material is generally selected as the more radiation-responsive material and can be selected to be a neutron-responsive or gamma-responsive material, based on the particular type of application and reactor.
The emitter material selected for use in self-powered radiation detectors disposed in-core of a nuclear reactor must meet stringent mechanical and nuclear considerations. Some of the properties which are desirable are good mechanical ductility, a high melting temperature, and desirable neutron cross section and/or gamma ray interaction probability. Some of the more widely used emitter materials are rhodium and cobalt for neutron-responsive detectors, and platinum for gamma ray-responsive detectors. A metal which has a very desirable essentially solely gamma radiation response is lead, which however has not found application in such detectors because of the low melting point of lead which is about 327.degree. C.
It has been proposed that a self-powered detector be fabricated utilizing an emitter with an essentially pure gamma-radiation response characteristic in order to avoid the difficulty of interpreting a signal which is the sum of the response to both neutron and gamma radiation. The use of neutron-responsive emitter materials can also give rise to a change in detector signal levels with reactor operating time as a result of excessive burnup of the neutron-responsive material.
It is, therefore, generally desirable to produce a self-powered detector which has essentially a pure gamma response. A prior art attempt to produce a pure gamma-response detector made use of a nickel alloy steel such as Inconel, which is a trademark material of the International Nickel Company, as the emitter electrode with a platinum cladding about the Inconel emitter. This prior art detector produced a lowered neutron response as a result of the platinum cladding without significantly effecting the gamma response of the emitter material. This platinum-clad design, however, still produces a mixed response which is shifted to be more highly gamma responsive. It has been well known that lead exhibits a superior, essentially pure gamma response. However, the low melting point of lead has made it unsuitable for use as an emitter in a self-powered detector for use within a nuclear reactor core because of the high operating temperature within the core. The use of a lead alloy as a self-powered detector emitter material is set forth in U.S. Pat. No. 4,434,370 entitled "Self-Powered Radiation Detector with Improved Emitter", owned by the assignee of the present invention. The lead alloy proposed in the copending application included lead as the major constituent with a minor constituent selected from the group of metals of aluminum, copper, nickel, platinum, or zinc.